DolphinQt: Adjust variable name for Qt root directory in steamrt builds

[OatmealDome]

QT_DIR isn't defined when I attempt to create testing builds, but it works on the buildbot for whatever reason. Switching to Qt6_DIR fixes my local builds.

[AdmiralCurtiss]

I think this is right. I'm not sure where QT_DIR comes from but Qt6_DIR definitely set by find_package() since the Qt package in CMake is called Qt6 (see https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html#config-file-packages).